1. Field of the Invention
The present invention pertains to carbon fiber production from polyacrylic polymer and copolymer fibers in which an oiling agent is used.
2. Description of the Related Art
Carbon fibers, also sometimes termed “graphite” fibers, are well known reinforcing agents in a variety of both inorganic and polymer composites, particularly the latter. Carbon fibers are prepared by the thermolytic treatment of organic substrates in fiber form. While there are numerous substrates from which carbon fibers may be formed, the predominant precursor fibers are pitch-based fibers and organic fibers, principally polyacrylic fibers, and in particular polyacrylonitrile (“PAN”) homopolymer and copolymer fibers. Pitch-based fibers are comparatively less expensive, and have high modulus. However, their bending strength and tensile strength are low compared to PAN-based fibers. As used herein, carbon fiber precursor, “CFP” also include PAN-based fibers as well as other acrylic fibers suitable for carbon fiber production.
PAN-based carbon fibers are generally prepared by first providing PAN fibers. These fibers may be prepared by numerous processes, but most of these involve providing a high solids solution of the CFP in a suitable solvent such as dimethylsulfoxide, extruding this solution through a spinneret, and coagulation in an aqueous bath, which may also contain a water miscible organic solvent.
The coagulated fiber thus obtained is generally washed, and then drawn in hot water. An oiling agent is added to the fiber, generally after drawing, for further processing. The fibers, now coated with oiling agent, are heat treated to dry the fibers, and further drawn at high temperatures, for example using steam under pressure. The fibers are then oxidized at elevated temperature under tension in an oxidizing atmosphere, generally air, for example at 200° C. to 400° C. Following oxidation, the fibers are carbonized at increasing temperatures in the range of, for example, 400° C. to 1300° C. in a substantially non-oxidizing atmosphere, and graphitized at a temperature exceeding 2000° C. An example of such a process is disclosed in U.S. Pat. No. 5,269,984, which is incorporated herein by reference, as is also U.S. Pat. No. 4,698,413, which discloses a dry spinning process. U.S. Published Statutory Invention Registration H1052 (May 1992) discloses an improvement in these processes by including ammonia in the oxidizing atmosphere.
The fibers are often treated in the form of a fiber bundle or “tow.” However, fusion may occur between single fibers in the stabilization step of converting the precursor fiber bundle into a stabilized fiber bundle, wherein the fusion may cause process failure such as fluffing and bundle breakage in the stabilization step and the subsequent carbonization step. It is known that applying an oiling agent to the precursor fiber bundle is important in avoiding this fusion, and a large number of oiling agent compositions have been utilized. For example, a silicone-based oiling agent in which an amino-modified silicone, an epoxy-modified silicone, a polyether-modified silicone or the like is frequently used as an oiling agent composition, due to both high heat resistance and effective suppression of fusion.
However, for silicone-based oiling agents composed mainly of silicone compounds, the silicone component undergoes a crosslinking reaction upon heating, resulting in an increase in viscosity. As a result, a viscous material derived therefrom may accumulate on the surfaces of fiber transporting rollers and guides in the precursor fiber bundle production process and in the stabilization step, and fiber bundles may be wound around or be caught in the rollers and guides, resulting in thread breakage, thereby leading to reduction in operability. Moreover, in oiling agent compositions containing silicone compounds, decomposition of the latter may produce silicon compounds such as silicon oxide, silicon carbide and silicon nitride in the heating step, and the scale thereby formed reduces the stability of the heating step and the quality of the product.
For this reason, non-silicone-based oiling agents have been proposed for many years for improving the operability of the heating step. Examples of non-silicone-based oiling agents include polybutenes, a blend of a polyoxyethylene higher aliphatic alkyl ethers, and an antioxidant, neopentyl alcohol derivatives, alkyl or alkenyl thio fatty acid esters polymeric amide compounds, ammonium salts of a fatty acid esters, fluorochemical surfactants, and aromatic esters and amides.
However, although non-silicone-based oiling agents have advantages such as no formation of silicon compounds in the heating step and use of inexpensive raw materials, these oiling agents are often poorer in thermal stability than silicone-based oiling agents, which causes fluffing and bundle breakage due to the fusion in the heating step. In addition, since the mechanical properties of the product carbon fiber bundle are also poorer than those produced with a silicone-based oiling agent, the use of non-silicone-based oiling agents for acrylic precursor fibers for carbon fibers is limited to a limited range of product classes.
It has also been proposed to reduce silicon compounds produced in the heating step derived from a silicone-based components by combining a silicone-based oiling agent and a non-silicone-based oiling agent. However, this technique is problematic in that the compatibility of silicone compounds with non-silicone compounds is low, and thus it is impossible to uniformly adhere a mixture of the silicone compound and the non-silicone compound to the surface of the precursor fiber bundle. As a result, prevention of fusion between single fibers has been insufficient where the non-silicone compound is unevenly distributed, e.g. where the silicone component is present in a small amount or is not substantially present, and it is thus difficult to stably obtain a carbon fiber bundle with excellent mechanical properties.
Furthermore, it has been proposed improve emulsion stability of the oiling agent by adding an alkylene oxide-modified silicone to an oiling agent containing a silicone and a non-silicone component. However, although an alkylene oxide-modified silicone has some stabilizing effect on the emulsion, the compatibility of the silicone and non-silicone components is still insufficient. As a result, adhesion of the oiling agent component to the precursor fiber bundle is not uniform, and fusion between single fibers cannot be completely prevented. Therefore, it has been difficult to stably obtain a carbon fiber bundle with excellent mechanical properties.
Thus, with respect to process stability and development of mechanical properties of carbon fiber bundles, the use of only non-silicone-based oiling agent compositions tends to be poorer than the use of an oiling agent composition using a silicone compound as the main component. Therefore, a high-quality carbon fiber bundle cannot be stably obtained. Further, when an oiling agent composition having a reduced content of silicone compound is used, it is difficult to uniformly adhere the silicone compound and the non-silicone compound to the surface of the precursor fiber bundle. Therefore, again, a high-quality carbon fiber bundle cannot be stably obtained. Thus, the problem of decreased operability due to formation of silicon compounds in the heating step stemming from a silicone-based oiling agent and the problem of reduction of mechanical properties of the carbon fiber bundle due to a non-silicone-based oiling agent are inextricably linked, and both of these problems have not been solved by the art.
In addition to preventing fusion, the oiling agent also provides lubrication, preventing snagging and breaking of fibers as they are drawn, and as they pass through the process to the carbonizing furnace. As indicated previously, numerous fiber finishes have been used in the past, but the selection of a suitable fiber finish is not straight forward, as first, the oxidized fibers have much different properties from the non-oxidized fibers; the fiber finish must be able to withstand the high temperature oxidative environment of the oxidizing furnace; and must not interfere with graphitic orientation nor the carbonization in the carbonizing furnace. Thus, many textile “oils” which include polyoxyalkylene polyethers, solutions and dispersions of waxes, and conventional silicones, including aminoalkyl-functional silicones and polyether silicones have not provided the desired results, or are in need of improvement. In the U.S. Pat. No. 5,269,984, for example, aminoalkyl-terminated organopolysiloxanes (“amine oils”) are disclosed as an “oiling agent”, and such amine oils continue to be used today. Such oils must generally be applied neat or dissolved in organic solvent. When supplied as a dispersion, a relatively large amount of emulsifier must be used to provide a stable dispersion (emulsion). The large amount of emulsifier may interfere with the oxidation and carbonization processes.
It would be desirable to provide a “robust” oiling agent for use in the production of carbon fibers from acrylic fibers, and particularly PAN-based organic fibers, which provides a stable and easily preparable emulsion, and which is compatible with additional silicone and non-silicone oiling agents and oiling agent components.